A summary of all the facilities provided by the telephone system is given below:
1. Dial-up telephone lines using the standard switching circuits. Over long distances the connections may use copper wires, Optical Fibre or microwave. Generally, the circuits are analogue although they are gradually being converted to digital circuits. In India, normally in dial-up lines the bandwidth available is 9600 bps and dial-up lines are used where data transmission may be important but not time critical.
2. Leased lines are used by organisations who need to transfer data on-line and where the data has to be transferred regularly and frequently.
The volume and frequency of the data to be transferred as well as the availability decides the bandwidth to be used. In India, leased lines are currently available in 9600 bps, 64 Kbps and 2 Mbps bandwidths. The cost of hiring leased lines in India were fairly high till recently when their rates have been substantially reduced by anything between 40% and 90%.
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Normally, leased lines in India utilise circuit switching, but over long distances they may use Fibre Optic or microwave transmission. Also, partly digital circuits may be used. The decision whether to use a leased line or not and what is the bandwidth to use depends upon the volume, frequency and speed with which data transfer must be done as also upon the availability of leased lines and bandwidth.
An example of the use of leased lines could be in banking transactions, where on-line data transfer may be a necessity if the bank wishes to maintain good customer services and keep the cases of fraud under check.
There may be other examples such as a steel plant keeping track of it stocks in different stockyards throughout the country and keeping their financial accounts centralised by recording their financial transactions on-line at all their offices and stockyards throughout the country. In each case, while leased lines may be definitely required, the bandwidth to opt for will depend upon the frequency and volume of the data.
3. Narrowband Integrated Systems Digital Networks, or narrowband ISDN as it is normally referred to, is a new addition to telephone services in India. Currently, narrowband ISDN is being offered for data transmission between fourteen cities in India. It is available in bandwidths of 64 Kbps and 2 Mbps. ISDN services make extensive use of Fibre Optics in the local loop.
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The chief characteristics of narrowband ISDN are the exceptional quality of the transmission and the high bandwidth offered. Most of the narrowband ISDN network in India is digital. Although the use of narrowband ISDN is not yet very popular in India, the high bandwidth offered by it as well as the quality of the lines and low error levels are likely to increase its popularity quite rapidly.
This fact is obvious from the recent action of the department of telephones in India. In this, they have recommended that the rates be reduced for leased lines by up to 90% because they are not able to compete with the narrowband ISDN services being also offered by the department of telephones. We shall look into the technology of both narrowband ISDN as well as broadband ISDN a little later.
4. Broadband ISDN has recently been made available in India yet but with the rapid improvements in the infrastructure facilities in India and serious efforts to improve telephone facilities it is bound to flourish. In fact, the introduction of narrowband ISDN in India was a low-cost stopgap improvement and having been found successful, broadband ISDN was introduced.
Testing, for introduction of broadband ISDN in India, was done before it was introduced. Broadband ISDN is characterised by the same level of quality as narrowband ISDN and even higher bandwidth.